The freezing point depression of the solution or pure substance that is added with the solvent is calculated through the equation,
ΔTf = Kfm
where ΔT is the freezing point depression, Kf is the constant for water given to be -1.86°C/m and m is the molality of the solution.
Molality is calculated through the equation,
m = number of moles solute/ kg of solvent
Calculation of molality is shown below.
m = (21.5 g C6H12O6)(1 mol/180 g) / (0.255 kg)
m = 0.468 molal
The freezing point depression is then,
ΔTf = (-1.86°C/m)(0.468 m) = -0.87°C
Answer: -0.87°C
